DE102005059314B4 - Acid, chromium-free aqueous solution, its concentrate, and a process for the corrosion protection treatment of metal surfaces - Google Patents

Abstract

Acid, chromium-free aqueous solution of a fluoro-complex of at least one element M selected from the group B, Si, Ti, Zr and Hf having a pH in the range from 2 to 5.5 for the treatment of metal surfaces, characterized in that they additionally a b) one or more components selected from: tin ions, bismuths, pH 2.5 to 5.5 buffer systems, aromatic carboxylic acids having at least two groups containing donor atoms, or derivatives of such carboxylic acids , c) contains copper ions and / or silver ions.

Description

The present invention relates to a new product for anticorrosive treatment of metal surfaces. Furthermore, a special process for corrosion protection treatment especially of steel surfaces is described. The corrosion protection treatment is intended above all as a pretreatment for a subsequent cathodic electrodeposition coating.

Anticorrosion agents, which are an acidic aqueous solution of fluoro complexes, have long been known. They are increasingly used as a replacement for chromating, which are increasingly less used because of the toxicological properties of chromium compounds. As a rule, such solutions of fluoro-complexes contain further anti-corrosive agents that further improve the anti-corrosion effect and paint adhesion.

For example, this describes DE 19 33 013 A in one embodiment, a treating solution which is an aqueous solution of ammonium hexafluorozirconate, sodium nitrate, cobalt nitrate, and sodium m-nitrobenzenesulfonate and has a pH of 5.2. The solution can be used to treat zinc, steel or aluminum surfaces. EP 1 571 237 A1 describes a treatment solution and treatment method for iron, zinc, aluminum and magnesium containing surfaces. This solution has a pH in the range of 2 to 6 and contains 5 to 5,000 ppm of zirconium and / or titanium and 0.1 to 100 ppm of free fluoride. In addition, the solution may contain further components selected from chlorate, bromate, nitrite, nitrate, permanganate, vanadate, hydrogen peroxide, tungstate, molybdate or in each case the associated acids. Organic polymers may also be present. After treatment with such a solution, the metal surfaces can be rinsed with another passivating solution. EP 1 405 933 A1 discloses a composition for treating surfaces of iron and / or zinc containing at least one of Ti, Zr, Hf and Si, and a fluorine ion source, with certain conditions being set to the concentration ratios of these two components. In addition, this solution may contain the same additional active ingredients as in EP 1 571 237 A1 contain. As further components, metal ions selected from the group Ag, Al, Cu, Fe, Mn, Mg, Ni, Co and Zn can be present. DE 100 10 758 A1 discloses a treatment solution especially for surfaces of zinc, aluminum and / or magnesium containing complex fluorides of Ti, Zr, Hf, Si and / or B as well as organic polymers. Additionally, the solution may contain ions of one or more of the metals Mn, Ce, Li, V, W, Mo, Mg, Zn, Co, and Ni. Further potential additives are compounds which are known in the case of layer-forming phosphation as phosphatization accelerators. WO 95/14539 A1 describes treatment solutions for metal surfaces, in particular for aluminum, containing complex fluorides of Ti, Zr, Hf, Si, Ge, Sn or B as well as organic hydroxycarboxylic acids having at least 2 hydroxyl groups (wherein the hydroxyl groups of the carboxylic acid groups are not counted) per carboxylic acid group. A specific example of such an acid is gluconic acid.

In all of these documents, either generally in the specification or specifically in the embodiments, it is mentioned that the metal surfaces treated with said solution are dried before being coated with another organic coating such as a varnish. Especially in the manufacture, pretreatment and painting of automobile bodies, however, it is desirable, because of the short cycle times and as short a length as possible of the pretreatment line, that the pretreated bodies without intermediate drying, i. when wet, retract into the dip bath. A drying step in which the pretreatment layer can be chemically modified and / or solidified by dewatering, for example, is undesirable. Therefore, in the pretreatment, a corrosion-protective layer must be produced, which has the required effect for corrosion protection and paint adhesion without drying before applying a first organic paint layer.

A coating process that meets these requirements is in EP 433 876 A1 described. The treatment of, for example, surfaces of steel, zinc and aluminum is carried out here with a solution containing Zr, Ti and / or Hf and fluoride ions and additionally a water-soluble resin, which can be regarded as polyvinyl or polyallylamine. In the only embodiment which describes painting of the wet-treated pretreated metal surface, an aqueous solution of fluorozirconic acid, polyallylamine, zinc nitrate, silicic acid and ascorbic acid is used. Here it can be expected that the organic polymer is an essential component for the corrosion protection effect and the paint adhesion. However, the presence of organic polymers in pretreatment solutions is often undesirable because they can lead to difficulties in wastewater treatment.

Surfaces of zinc, aluminum and galvanized steel can be pretreated with very differently composed aqueous solutions of fluoro-complexes, wherein the above Requirements can be met. In the case of steel surfaces which have not been precoated, the stated requirements can hitherto essentially be fulfilled only with a layer-forming zinc phosphating. Compared to the standard of zinc phosphatizing coatings, pretreatment with aqueous solutions of fluoro-complexes shows significant disadvantages in terms of anticorrosive effect and paint adhesion if the pretreated surfaces are not dried before painting.

The EP 1 433 877 A1 discloses a pretreatment method for the metals iron, zinc and aluminum based on acidic aqueous treatment solutions comprising, in addition to compounds of the elements Zr, Ti and Hf and the element fluoro organic aminosilanes and optionally other corrosion and adhesion enhancing constituents such as ions of the elements Mg, Zn, Ca, Al, Ga, In and Cu. In the investigations that led to the underlying invention, it became clear that the addition of one or more components selected from tin ions, bismuth ions, buffer systems for the pH range from 2.5 to 5.5, aromatic carboxylic acids or derivatives thereof, the corrosion protection effect of aqueous solutions of fluoro-complexes for surfaces of steel, aluminum, zinc and galvanized steel generally improved. This is true regardless of whether or not the surfaces are dried between contact with this solution and a subsequent finish. Such treatment solutions show a positive effect for corrosion protection and paint adhesion of metal surfaces in general.

1. a) sowohl Zinkionen als auch Magnesiumionen,
2. b) eine oder mehrere Komponenten ausgewählt aus: Zinnionen, Wismutionen, Puffersystemen für den pH-Bereich von 2,5 bis 5,5, aromatischen Carbonsäuren mit mindestens zwei Gruppen, die Donoratome enthalten, oder Derivaten solcher Carbonsäuren,
3. c) Kupferionen und/oder Silberionen enthält.

The present invention is thus characterized in the provision of an acidic, chromium-free aqueous solution of at least one element M selected from the group B, Si, Ti, Zr and Hf having a pH in the range from 2 to 5.5 for the treatment of metal surfaces that in addition

1. a) both zinc ions and magnesium ions,
2. b) one or more components selected from: tin ions, bismuth ions, buffer systems for the pH range from 2.5 to 5.5, aromatic carboxylic acids having at least two groups containing donor atoms, or derivatives of such carboxylic acids,
3. c) copper ions and / or silver ions.

As one of the above-mentioned components, the treating solution may contain aromatic carboxylic acids characterized by having at least 2 groups in the molecule containing donor atoms. Donor atoms are those atoms that carry lone pairs of electrons through which they can coordinate to transition metal ions. Typical donor atoms are oxygen, nitrogen and sulfur atoms. Thus, the carboxyl group of the aromatic carboxylic acids is itself already a group containing donor atoms. An aromatic carboxylic acid whose molecule has at least 2 carboxyl groups thus falls under the definition mentioned. Furthermore, these include those aromatic carboxylic acids which bear, in addition to the carboxyl group, for example at least one hydroxyl group, at least one amino group or at least one nitro group. Examples of such carboxylic acids are the different positional isomers of benzenedicarboxylic acid, in particular phthalic acid or the different position isomers of hydroxy, amino or nitrobenzoic acid.

In this case, preference is generally given to those aromatic carboxylic acids in which at least 2 groups which contain donor atoms are arranged in such a way that 5, 6 or 7-membered chelate complexes with transition metal ions can be formed via the donor atoms. Accordingly, particularly preferred aromatic carboxylic acids are: phthalic acid, salicylic acid, o-aminobenzoic acid or o-nitrobenzoic acid. Instead of aromatic carboxylic acids containing only a single benzene ring, the corresponding acids with fused ring systems, for example, the naphthalene or anthracene-derived acids can be used.

It is also possible to use derivatives of the carboxylic acids mentioned. These are understood as meaning those molecules in which one or more hydrogen atoms of the basic form (for example hydrogen atoms on the aromatic nucleus, hydrogen atoms of the hydroxyl or amino groups or hydrogen atoms of the carboxylic acid groups) have been replaced by other atoms or atomic groups.

For toxicological reasons, an aqueous solution according to the invention is used which is substantially free of chromium (VI) compounds and which preferably contains no chromium compounds at all. This leaves traces of chromium compounds unconsidered, which can reach the treatment solution by being dissolved out of stainless steel containers. As "chromium-free" in this sense, treatment solutions are understood to contain not more than 1 ppm, in particular not more than 0.1 ppm chromium. The treatment solutions according to the invention do not represent phosphating solutions, ie they do not lead to the formation of an amorphous or crystalline phosphate layer. This is achieved by the fact that Treatment solutions preferably not more than 1 g / l of inorganic phosphates or phosphoric acid, calculated as PO ₄ ^3- , included.

However, phosphate contents in the range of, for example, 10 to 500 mg / l are tolerable and may even improve the effect of the treatment solution.

The acidic treatment solution according to the invention has a pH in the range from 2 to 5.5, in particular from 3.5 to 5. Preferably, the pH is adjusted to the said acidic range by using the fluoro-complex at least partially in the form of an acid. However, it can also be adjusted by another acid, for example nitric acid.

It is provided according to the invention that the aqueous solution contains in addition to at least one of said essential components (tin ions, bismuthions, buffer systems for the pH range from 2.5 to 5.5, aromatic carboxylic acids or derivatives thereof) both zinc ions and magnesium ions, and the presence of copper and / or silver ions is required.

• Zinnionen: 1 bis 2000 mg/l, vorzugsweise 5 bis 500 mg/l,
• Wismutionen: 1 bis 2000, vorzugsweise 5 bis 500 mg/l,
• Puffersystem für den pH-Bereich von 2,5 bis 5,5: in einer solchen Menge, dass sich der pH-Wert der Lösung bei einem Eintrag von einem Val Säure oder Lauge pro Liter Lösung um nicht mehr als 0,2 Einheiten verändert,
• aromatische Carbonsäuren: 0,1 bis 1000, vorzugsweise 1 bis 500 mg/l.

The one or more essential components are preferably present in the following concentration ranges:

• Tin ions: 1 to 2000 mg / l, preferably 5 to 500 mg / l,
• Bismuths: 1 to 2000, preferably 5 to 500 mg / l,
• Buffering system for the pH range from 2.5 to 5.5: in such an amount that the pH of the solution does not change by more than 0.2 units given an entry of one Val acid or lye per liter of solution,
• aromatic carboxylic acids: 0.1 to 1000, preferably 1 to 500 mg / l.

• Nitrationen: 0,1 bis 5000 mg/l, vorzugsweise 1 bis 1000 mg/l,
• Kupfer-, Cobalt-, Nickel- und/oder Silberionen: jeweils 0,1 bis 300 mg/l, vorzugsweise1 bis 30 mg/l,
• Vanadium- oder Vanadationen: 1 bis 2000, vorzugsweise 5 bis 500 mg/l (als Vanadium berechnet),
• Magnesiumionen: 1 bis 2000, vorzugsweise 5 bis 500 mg/l,
• Manganionen: 1 bis 2000 mg/l, vorzugsweise 5 bis 500 mg/l,
• Zinkionen: 1 bis 2000, vorzugsweise 5 bis 500 mg/l,

The following are optional for the following optional or according to the invention necessarily contained in the aqueous solution:

• Nitrate ions: 0.1 to 5000 mg / l, preferably 1 to 1000 mg / l,
• Copper, cobalt, nickel and / or silver ions: in each case 0.1 to 300 mg / l, preferably 1 to 30 mg / l,
• Vanadium or vanadium ions: 1 to 2000, preferably 5 to 500 mg / l (calculated as vanadium),
• Magnesium ions: 1 to 2000, preferably 5 to 500 mg / l,
• Manganese ions: 1 to 2000 mg / l, preferably 5 to 500 mg / l,
• Zinc ions: 1 to 2000, preferably 5 to 500 mg / l,

As a buffer system for said pH range, an acetic acid / acetate buffer is particularly suitable. Another suitable buffer system based on potassium hydrogen phthalate.

The aqueous solution according to the invention preferably contains an amount of fluoro-complex such that the concentration of the metal M is in the range from 1 to 5,000 mg / l, preferably in the range from 5 to 1,000 mg / l and in particular in the range from 10 to 500 mg / l is. As metal M zirconium and / or titanium is particularly preferred.

Furthermore, it is preferred that in the fluoro-complex the element M is selected from the group Si, Ti, Zr and Hf and that the aqueous solution contains on average at least 1, preferably at least 3, in particular at least 5 fluorine ions per ion of the element M. , In this case, the term "on average" means the calculated atomic ratio of fluorine ions to M ions in the aqueous solution. Because of the stability of fluoro complexes of the said metals M, it is to be expected that when the aqueous solution of the invention contains less than 6 fluorine ions per M ion, the fluorine ions are almost completely bound to the M ions in the form of fluoro complexes , In this case, therefore, the fluoride should be substantially completely present as a "complex fluoride".

However, the aqueous solution according to the invention may also contain more fluoride ions than is required for complete formation of hexafluoro-complexes. In this case, it can be assumed that there are 6 fluorine ions per M ion in the fluoro complex and that the excess fluoride ions are present as so-called "free fluoride". This can be for example in the form of HF and / or water-soluble Salts thereof are the case. This embodiment of the present invention is characterized in that 6 fluorine ions per ion of the metal M (M selected from Si, Ti, Zr, Hf) are present in the fluoro complex and that the aqueous solution additionally contains 1 to 1000 mg / l fluoride ions that are not bound to zirconium.

If in the context of the present invention an "acid", especially of "carboxylic acid" is mentioned, this is to be understood as the free acid and / or their anions. The skilled person is aware that regardless of whether the acid is used in free form or in the form of salts soluble in the indicated concentration range, an equilibrium between free acid and salt form, which depends on the pKs of the acid in question and the pH Value of the aqueous solution. Concentration data are calculated as the free acid. The same applies, for example, to H ₂ ZrF ₆ or other acids which are present in the aqueous solution.

In a preferred embodiment, the aqueous solution according to the invention contains at least one aromatic carboxylic acid, preferably salicylic acid, or derivatives thereof. This may be present together with the bismuths and / or the buffer system. It is preferably present in a concentration of at least 0.1 mg / l, preferably at least 1 mg / l and especially at least 10 mg / l. The upper limit of concentration is again more economic than technical. For example, as the upper limit of the concentration of the aromatic hydroxycarboxylic acid 1000 mg / l, preferably 500 mg / l and in particular 400 mg / l can be selected.

In a further preferred embodiment, the aqueous solution according to the invention contains a buffer system for the pH range from 2.5 to 5.5, as described above.

Furthermore, the aqueous solution according to the invention may additionally contain aluminum ions. These can be introduced in the form of soluble salts, for example in the form of the nitrates. In this case, the aqueous solution according to the invention preferably contains 1 to 1000 mg / l, in particular 10 to 500 mg / l of aluminum ions. Aluminum ions can serve as "scavengers" for excess free fluoride ions as they form stable fluoro complexes with them. Free fluoride ions are formed in the aqueous solution according to the invention in that the metal M, for example zirconium, presumably in the form of oxides, is deposited on the treated metal surfaces. The fluoride ions originally bound to the metal M are thereby released. The free-fluoride ionization action of the aqueous solution can be reduced by the presence of the aluminum ions due to complex formation.

• 0,05 bis 2 g/l m-Nitrobenzolsulfonationen,
• 0,1 bis 10 g/l Hydroxylamin in freier oder gebundener Form,
• 0,05 bis 2 g/l m-Nitrobenzoationen,
• 0,05 bis 2 g/l p-Nitrophenol,
• 1 bis 70 mg/l Wasserstoffperoxid in freier oder gebundener Form,
• 0,05 bis 10 g/l organische N-Oxide
• 0,1 bis 3 g/l Nitroguanidin
• 1 bis 500 mg/l Nitritionen
• 0,5 bis 5 g/l Chlorationen.

In addition to the components already mentioned, the aqueous solution according to the invention may contain compounds which are used in the layer-forming phosphating process as so-called "accelerators". These accelerators have the property of trapping hydrogen atoms which form during the pickling attack of the acid on the metal surface. This reaction, also referred to as "depolarization", facilitates the attack of the acidic treatment solution on the metal surface and thereby accelerates the formation of the corrosion protection layer. For example, accelerators can be used, which in the above-mentioned document DE 199 33 189 A1 are listed:

• 0.05 to 2 g / l of m-nitrobenzenesulfonate ions,
• 0.1 to 10 g / l of hydroxylamine in free or bound form,
• 0.05 to 2 g / l m-nitrobenzoate ions,
• 0.05 to 2 g / l p-nitrophenol,
• 1 to 70 mg / l of hydrogen peroxide in free or bound form,
• 0.05 to 10 g / l of organic N-oxides
• 0.1 to 3 g / l nitroguanidine
• 1 to 500 mg / l nitrite ion
• 0.5 to 5 g / l of chlorate ion.

1. a) Polymere oder Copolymere von ungesättigten Alkoholen oder deren Ester oder Ether,
2. b) Polymere oder Copolymere von ungesättigten Carbonsäuren, Organophosphonsäuren, Organophosphinsäuren oder jeweils deren Salze, Ester oder Amide,
3. c) Polyaminosäuren oder Proteine oder jeweils deren Salze, Ester oder Amide,
4. d) Kohlehydrate oder deren Ester (inklusive Xanthogensäure-Ester) oder Ether,
5. e) Polyamine, bei denen die Stickstoffatome in die Polymerkette eingebunden sind,
6. f) Polyether,
7. g) Polyvinylphenole und deren Substitutionsprodukte,
8. h) Epoxidharze, Aminoplastharze, Tannine, Phenol-Formaldehydharze,
9. i) Polymere und Copolymere von Vinylpyrrolidon.

Furthermore, the aqueous solution of the invention may contain organic polymers whose positive effect is known in a corrosion protection treatment. Examples of such polymers are:

1. a) polymers or copolymers of unsaturated alcohols or their esters or ethers,
2. b) Polymers or copolymers of unsaturated carboxylic acids, organophosphonic acids, organophosphinic acids or in each case their salts, esters or amides,
3. c) polyamino acids or proteins or in each case their salts, esters or amides,
4. d) carbohydrates or their esters (including xanthogen acid esters) or ethers,
5. e) polyamines in which the nitrogen atoms are incorporated in the polymer chain,
6. f) polyether,
7. g) polyvinylphenols and their substitution products,
8. h) epoxy resins, aminoplast resins, tannins, phenol-formaldehyde resins,
9. i) Polymers and copolymers of vinylpyrrolidone.

If such polymers are present, their concentration in the aqueous solution according to the invention is preferably below 2000 mg / l. For secondary technical reasons, for example to simplify wastewater treatment, it may be advantageous to substantially or completely dispense with the presence of organic polymers in the aqueous solution according to the invention. Thus, a preferred embodiment of the present invention is characterized in that the aqueous solution does not contain more than 1 mg / L of organic polymer. Under this condition, it is further preferred that the inventive aqueous solution additionally contains 10 to 1000 mg / l, preferably 50 to 500 mg / l of silicon in the form of silica particles having a mean particle size below 1 micron. The abovementioned silica particles having an average particle size of less than 1 μm are known to the person skilled in the art under different generic names. For example, they are referred to as colloidal silica, precipitated silica or fumed silica. The average particle size, which is preferably in the range from about 0.01 μm to about 1 μm, can be determined by light diffraction methods or by electron microscopy.

The treatment solution according to the present invention can be prepared on-site by dissolving the above components in water and adjusting the pH. However, this procedure is unusual in practice.

Instead, in practice aqueous concentrates are usually made available, from which the ready-to-use aqueous solution according to the invention is prepared on site by dilution with water and, if necessary, adjustment of the pH. Accordingly, an aqueous concentrate which when diluted with water includes a factor of from about 10 to about 100, more preferably a factor in the range of from about 20 to about 50 and, if necessary, adjusting the pH, comprises an acidic, chromium-free, aqueous solution of fluorochemicals. Complexes according to the above description of the invention, also forms the subject of the present invention.

The concentrates according to the invention may contain polymers with thickening and / or dispersing properties for stabilization. Examples of such polymers are polymers or copolymers of unsaturated carboxylic acids, carbohydrates or proteins. These can be present in a concentration of up to 50 g / l.

For reasons of stability, such concentrates according to the invention are often adjusted so that, when diluted with water, the pH is not directly within the required range. In this case, after diluting with water, adjust the pH either down or up. Downward adjustment is by addition of an acid, in which case either the acid form of the fluoro-complex of the metal M or nitric acid is available. An adjustment of the pH value can be carried out with any basic substance, for example with a solution of alkali metal hydroxides or carbonates, ammonia or organic amines. However, to increase the pH, it is also possible to use basic compounds or salts, such as, for example, oxides, hydroxides or carbonates of metals, which are possible active components in the treatment solution. For example, for oxides, hydroxides or carbonates of magnesium or zinc can be used here.

The present invention further includes a process for anticorrosive treatment of bare metal surfaces by contacting the metal surfaces with an aqueous solution of the invention as described above.

It may be preferred that the metal surface is rinsed after contact with the aqueous solution according to the invention with an aqueous solution containing one or more components selected from compounds or salts of the elements cobalt, nickel, tin, copper, titanium and zirconium and / or of water-soluble or water-dispersible organic polymers.

By "bare" metal surfaces are meant metal surfaces that do not yet carry a corrosion-protective coating. Thus, the method according to the invention is the first or only treatment step which produces a corrosion protection layer, which in turn can serve as the basis for a subsequent coating. It is therefore not a post-treatment of a previously generated corrosion protection layer such as a phosphate layer.

For the treatment of the metal surface with the aqueous solution according to the invention, the general rule is:

The bare metal surface is brought into contact with the acidic aqueous solution according to the invention for a time in the range from 0.5 to 10 minutes, preferably in the range from 1 to 5 minutes. This can be done for example by immersion in the treatment solution or by spraying with the treatment solution. The temperature of the aqueous solution according to the invention is preferably in the range from 15 to 60 ° C., in particular in the range from 25 to 50 ° C. After this contact is preferably rinsed with water, especially with demineralized water. This may optionally be followed by the rinsing described above. In this case, then rinsed again with water.

According to the invention, no measures are required and should preferably even be avoided by which the metal surface is dried after contact with the aqueous solution according to the invention and before coating with the cathodically depositable electrodeposition paint. However, inadvertent drying may occur during plant downtime when the treated metal surface, such as an automobile body or part thereof, is in the air between the aqueous solution of the fluoro-complex and the electrocoating bath. However, this unintentional drying is harmless. But you can also dry the treated metal surfaces before they are coated with a kahodischen electrocoating or with another paint such as a powder coating.

The following experiments exemplify the investigations that led to the present invention.

As a substrate for the following experiments, cold rolled steel test sheets were used, as used in the automotive industry. All process steps were carried out as a dipping process. Abbreviations: demineralised water = deionized water, RT = room temperature, min. = Minutes, cathodic electrocoating. Table 1: General procedure. process step Bath composition bath temperature PH value duration of treatment cleaning Applicant's Alkaline Cleaner: 3% Ridoline ^R 1562 + 0.3% Ridosol ^R 1561 60 ° C Alkaline 5 min. kitchen sink Process water RT 1 min. kitchen sink VE water RT 1 min. preparation According to tables (Zircon is used as H ₂ ZrF ₆ ) 30 ° C According to tables 3 min. kitchen sink VE water RT 0,5 min. Optional drying (see tables) drying cabinet 50 ° C 60 min. KTL Electrocoating "Cathoguard ^R" 310 from BASF Table 2: Bath composition for pretreatment in the process sequence according to Table 1 with drying after pretreatment and corrosion results: Intermediate climate test according to VDA 621415: Corrosion on the cut after 70 days in mm, and rockfall after 70 days, scale of 0.5-5 (the smaller, the better) Bath composition AC climate test: corrosion Wechselklimatest: rockfall Comparison 1: 3.4 4.5 150 mg / l Zr, pH 4, Example 1: 1.9 3.5 150 mg / l Zr + 50 mg / l salicylic acid, pH 4, Example 2: 1.6 4.0 150 mg / l Zr + 200 mg / l salicylic acid, pH 4, Comparison 2: 9.3 5.0 150 mg / l Zr + 200 mg / l citric acid, pH 4

Result:

Examples 1 and 2 demonstrate the beneficial effect of adding salicylic acid when the pretreatment layer is dried. In contrast, an addition of citric acid (comparison 2) has a rather negative effect. Table 3: Bath composition for pretreatment in the process sequence according to Table 1 without drying after pretreatment ("wet on wet") and corrosion results: Alternating climate test according to VDA 621415: corrosion at the cut after 35 days in mm. Bath composition AC climate test: corrosion Comparison 3: 150 mg / l Zr, pH 4 1.5 Example 3: 150 mg / l Zr, 400 mg / l nitrate (used as nitric acid), 200 mg / l Si (used as colloidal silicic acid), pH 4 1.3

Result:

Example 3 demonstrates the beneficial effect of adding nitrate and silica if the pretreatment layer is not dried.

Comparison 4, Example 4:

In the course of the process according to Table 1, the following treatment solution was used for the pretreatment (treatment time: 5 min.), On the one hand (comparison 4) being dried after the pretreatment and on the other hand (example 4) not being dried after pretreatment and before dip-coating: 150 mg / l Zr, 400 mg / l nitrate (used as nitric acid), 200 mg / l Si (used as colloidal silicic acid), pH 3.8.

Corrosion results: Interaction climate test according to VDA 621415: Corrosion at cut after 70 days in mm: Comparison 4 (with drying): 3.9 mm Example 4 (without drying): 3.6 mm

Result:

Example 4 shows that in the presence of nitrate and silica in the absence of drying better results are achieved than with drying. Table 5: Bath composition for pretreatment in the process sequence according to Table 5 without drying after pretreatment and corrosion results: Alternating climate test according to VDA 621415: Corrosion at cut after 70 days in mm, and rockfall after 70 days, scale from 0.5 to 5 (the smaller the better). Bath composition AC climate test: corrosion Wechselklimatest: rockfall Comparison 5 3.1 4.5 150 mg / l Zr, pH 4 Example 5 1.3 3.7 150 mg / l Zr + 20 mg / l Cu, pH 4 Example 6 1.1 3.2 150 mg / l Zr + 20 mg / l Cu + 200 mg / l Si, pH 4

Result:

Example 5 shows that the addition of 20 mm / l copper (as Cu (NO) ₃ in the "wet-on-wet" process provides significantly better undercrossing values in the alternating climate test.) Further addition of 200 mm / l silicon in the form of colloidal Silica (Example 6) gives a marked improvement in the K value in the stone impact test according to DIN 55996-1. Bath composition for pretreatment in the process sequence according to Table 1 without drying ("wet-on-wet") after pretreatment and corrosion results: Alternating climate test according to VDA 621415: Corrosion on the cut after 70 days in mm, and rockfall after 70 days according to DIN 559966-1, Scale from 0.5 to 5 (the smaller the better). Bath composition AC climate test: corrosion Wechselklimatest: rockfall Example 7 0.9 3.7 150 mg / l Zr + 5 mg / l Cu + 50 mg / l Si, pH 4 Example 8 0.7 2.3 150 mg / l Zr + 5 mg / l Cu + 50 mg / l Si + 50 mg nitroguanidine, pH 4

Result:

Examples 7 and 8 show in comparison that the addition of the accelerator nitroguanidine (50 mm / l) gives a further improvement in paint undercutting in the alternating climate test and in the combined cyclone and stone impact test.

Claims (13)

Acid, chromium-free aqueous solution of a fluoro-complex of at least one element M selected from the group B, Si, Ti, Zr and Hf with a pH in the range from 2 to 5.5 for the treatment of metal surfaces, characterized in that they additionally a) both zinc ions and magnesium ions, b) one or more components selected from tin ions, bismuthions, pH 2.5 to 5.5 buffer systems, aromatic carboxylic acids having at least two groups containing donor atoms, or derivatives of such carboxylic acids, c) copper ions and / or Contains silver ions. Aqueous solution after Claim 1 Characterized in that it contains an amount of fluoro complex in that the concentration of the element M in the range of 1 to 5000 mg / l, preferably in the range of 5 to 1000 mg / l and more particularly in the range of 10 to 500 mg / l is. Aqueous solution after one or both of Claims 1 and 2 , characterized in that in the fluoro complex, the element M is selected from the group Si, Ti, Zr and Hf and that the aqueous solution contains on average at least 1, preferably at least 3, in particular at least 5 fluorine ions per ion of the element M. Aqueous solution after Claim 3 , characterized in that there are six fluorine ions per ion of the element M in the fluoro-complex and that the aqueous solution additionally contains 1 to 1000 mg / l of fluoride ions which are not bound to the element M. Aqueous solution according to one or more of Claims 1 to 4 , characterized in that the aromatic carboxylic acid is selected from hydroxycarboxylic acids, aminocarboxylic acids, nitrocarboxylic acids and carboxylic acids having at least two carboxyl groups, or derivatives thereof. Aqueous solution according to one or more of Claims 1 to 5 , characterized in that it contains 0.1 to 1000 mg / l of said aromatic carboxylic acid, or derivatives thereof. Aqueous solution according to one or more of Claims 1 to 6 , characterized in that it additionally contains 0.1 to 5000 mg / l nitrate ions. Aqueous solution according to one or more of Claims 1 to 7 , characterized in that it contains 0.1 to 300 mg / l, preferably 1 to 30 mg / l copper and / or silver ions. Aqueous solution according to one or more of Claims 1 to 8th , characterized in that it contains not more than 1 mg / l of organic polymer. Aqueous solution after Claim 9 , characterized in that it additionally contains 10 to 1000 mg / l, preferably 50 to 500 mg / l silicon in the form of silica particles having a mean particle size below 1 micron. Aqueous concentrate which dilutes with water by a factor between 10 and 100 and, if necessary, adjusts the pH value of an aqueous solution according to one or more of the Claims 1 to 10 results. Process for the corrosion protection treatment of bare metal surfaces, characterized in that the metal surfaces are treated with an aqueous solution according to one or more of Claims 1 to 10 brings in contact. Method according to Claim 12 , characterized in that after contact with the aqueous solution of a fluoro-complex, the metal surface is rinsed with an aqueous solution which contains one or more components selected from compounds or salts of the elements cobalt, nickel, tin, copper, titanium and zirconium and / or from water-soluble or water-dispersible organic polymers.